Resolves #96

Adds capability to disable spectral display.

Adds right-click context
menu items to disable display and/or reenable display by selecting
another tuner.

Adds view menu items to disable spectral display or show a tuner.

Persists spectral display enabled state across application run sessions.

src/main/java/io/github/dsheirer/source/tuner/TunerEvent.java

@@ -29,6 +29,7 @@ public class TunerEvent
         CHANNEL_COUNT,
         FREQUENCY,
         SAMPLE_RATE,
+        CLEAR_MAIN_SPECTRAL_DISPLAY,
         REQUEST_MAIN_SPECTRAL_DISPLAY,
         REQUEST_NEW_SPECTRAL_DISPLAY;
     }